By David N. Leff
Most people probably relate the name "Lister" to an antiseptic mouthwash called Listerine. Infectious disease specialists identify that same name with Listeria monocytogenes, a deadly bacterium.
Both appelations recall Joseph, Lord Lister (1827-1912), the celebrated British trauma surgeon of the late 19th century, who founded the field of antiseptic medicine. To the hoots of his colleagues - who scorned the notion of infectious microorganisms - Lister in 1867 began swathing his open surgical incisions with bandages soaked in carbolic acid, a harsh disinfectant also known as phenol. Between that year and 1870 he treated 40 patients, only six of whom died. In the preceding three years, 1864-66, before he started carbolic acid antisepsis, 16 of 35 patients succumbed to their septic wounds.
There's a touch of poetic irony in the fact that Listerine's active ingredient is thymol, also a phenolic chemical. The mouthwash was invented in 1879 by Joseph Lawrence, a physician colleague of Lister's, according to Meghan Marschall, assistant director of marketing at Warner-Lambert, which manufactures over-the-counter Listerine. She added that Gerald Lambert, founder of Warner-Lambert, put it on the market in 1881 as a treatment for halitosis.
There's nothing poetic about Listeria monocytogenes, the pathogen that causes listeriosis. "It's a food-borne disease that's been much in the news lately," said microbiologist Daniel Portnoy, at the University of California, Berkeley. "It has caused outbreaks from contaminated cheese to cole slaw to - most recently - hot dogs." He said the organism "is ubiquitous. It can even grow in refrigerators, at temperatures just above freezing.
"L. monocytogenes causes more than 2,000 infections a year in the U.S. and 500 deaths," said Portnoy. "Though it hits immune-compromised people the hardest, its overall fatality rate is about 20 percent." The pathogen's lethal rap sheet in human disease includes meningitis, encephalitis, septicemia and endocarditis. It strikes mainly pregnant women, newborn infants and adults over 70.
Portnoy is senior author of a report in today's issue of Science, dated Nov. 3, 2000, titled: "A PEST-like sequence in listeriolysin O [LLO] essential for Listeria monocytogenes pathogenicity."
"We reported," he told BioWorld Today, "that this central virulence factor in Listeria, this highly toxic LLO hemolysin, contains a sequence of amino acids - PEST - that is absolutely essential for virulence. Also, that its mutated sequence is responsible for preventing toxicity to the host cell."
Is Microbial Altruism For Real?
A virulent microbe that spares its own host cell seems paradoxical. "Some people see it that way," Portnoy allowed. "It's really not a paradox. I tend to think the public appreciates that most pathogens don't kill their host, and an intracellular pathogen doesn't want to kill its host cell. It needs to live in it to survive. I appreciate that even scientists in the field have asked that question, so it's not naove."
One such scientist - molecular biologist Michael Zasloff, at Magainin Pharmaceuticals - answered the question of why a bacterium should be working against its own apparent interests this way: "As far as they're concerned," he told BioWorld Today, "we humans are irrelevant. We're so unimportant in this whole world that they're just waiting to get us anyway. They will be here far longer than we - along with animals and plants - will ever be."
PEST's 19-amino-acid sequence is rich in proline (P), glutamine (E), serine (S) and threonine (T). When the co-authors mutated that PEST sequence so the cell no longer recognized it, the mutant bacteria quickly killed off those host cells - their own source of survival.
"When we deleted the PEST sequence from the microorganism's chromosome," Portnoy noted, "the mutated protein remained biochemically active, yet the bacteria became 10,000-fold less virulent in mice than their native, wild-type counterparts. We showed that this mutant kills the host cell - the macrophage that Listeria enters." He added: "The macrophages can eat these organisms - that is, phagocytose them - but it doesn't kill them. What this meant is that if the intracellular pathogen kills its hospitable host cells, it's no longer pathogenic.
"Intracellular pathogens," Portnoy explained, "are among the leading causes of morbidity and mortality worldwide. The world's top three infectious killers of our time - HIV, TB and malaria - fall in this class. All are pathogens that ensconce themselves snugly inside their host cells and live to break out and wreak their havoc in neighboring cells."
Portnoy describes another, previously reported, protein-driven maneuver in Listeria's breakout. "Once the bacteria break free," he said, "they exploit an amazing host-cell mechanism of motility. They generate comet-like tails that push them around the macrophage like a speedboat. Eventually they slam into the cell membrane and pop out into the next cell, spreading their infection. What's elegant about this," he said, "is that over millions of years the bacteria have evolved individual proteins capable of exploiting complex processes that control host cell biology.
"Those three pathogens are very difficult to study," Portnoy noted, "which is why there are no effective vaccines for any of their diseases. So we decided to investigate a model system in Listeria that would allow us to answer the basic questions involved.
"There are three things one needs in order to study a pathogen effectively," he observed. "Ability to cultivate the organism; a way to manipulate it genetically and biochemically; and an animal model - or tissue. Take tuberculosis. It's an important pathogen, and a lot of researchers are making tremendous headway. But unlike Listeria, which forms a colony in 14 hours, it takes TB a month."
One Good Antigen For Vaccine Development
The co-authors envision their next listeriosis project as "maybe having to do with the human immune system." The LLO toxin, Portnoy pointed out, "happens to be an important antigen in the mouse, which probably relates it to vaccine development.
"The immunity to Listeria and other intracellular pathogens," he added, "is largely cell mediated. Cytotoxic T-cells recognize the infectious cells and kill them. The primary antigen in listeriosis is this same protein, LLO - and CTL T cells recognize the peptide derived from it. So we think that in the future this PEST sequence may help make other proteins more immunogenic for vaccines. And/or," he concluded, "it's going to be a signature molecule that maybe identifies other important antigens for other pathogens that are more important for vaccine development than Listeria."